I wanted to build a simple motion sensing programmable color changing light to use as a night light in the bathroom. I learned quite a bit about power management (or lack there of) in Arduino.
The other interesting thing I tried was to use a side-car processor to monitor and report back log data from the Arduino up to the web. I did this because I needed a way to debug my device and figure out how to tune it to work the way I wanted.
I built a small circuit to read log data from the primary circuit and shuttle it up to the cloud. This way I could monitor what the sensors and LED lights were doing. Here is some of the data graphed and as JSON